Each meal should consist of protein and two to three servings of vegetables preferably the green kind that has a lot of vitamins and helps with your digestion. You need to drink plenty of water to replenish what you lost during a workout. you can also drink milk and fruit juices are also good. You should not drink soft drinks or pop this will only add calories and has no nutritional value. Staying hydrated will help you flush out your muscles and keep you from getting dehydrated.
To work on gaining weight you should start resistance training. This forces the muscles in your body to work harder which increases your muscle size. you can try aerobics, cycling, running and weight lifting if you have the equipment at home or if you have a gym to work on in. To have a decent weight gain you need to work out about one hour a day. For you body and muscles to recover you need to select different parts of the body to work. This way you will start gaining weight as your muscles grow. It may take some time to gain your ideal weight if you are underweight you will need to be patient.